A liquid-crystal polymer (LCP) capable of forming an anisotropic molten phase is a thermoplastic resin having numerous properties such as high strength, high rigidity, high thermal resistance, mold processability. However, it makes a difference in molding shrinkage and mechanical properties between a molecular chain orientation direction and a transverse direction, and further, it has commercial disadvantages such as high cost.
On the other hand, a thermoplastic polyester resin such as polycarbonate resin (PC), which dose not form an anisotropic molten phase, is relatively inexpensive, but there is a disadvantage that it is inferior to LCP in the physical properties such as heat resistance and rigidity. Especially, since the resin lacks enough rigidity to use for a thin-film housing, it must be thickened on a design. Therefore, there is a limit in its applicability to recent miniaturization/lightening in the fields of electric, electronic, and communication apparatus.
Many trials have, then, been made to make the most of the advantages of LCP and thermoplastic resin not forming an anisotropic molten phase while making up the disadvantages of both by mixing and using them. However, an injection-molded article of a resin composition obtained by simply blending a thermoplastic resin with a small amount of LCP, cannot utilize the LCP's properties such as high strength, high rigidity, thermal resistance and moldability, and results in remarkable deterioration of the mechanical strength. This is because the exhibition of LCP's high mechanical properties and the like originates from a molecular orientation formed under shear and stretch stresses during melt processing. In the resin composition obtained by simply blending a thermoplastic resin and a small amount of LCP, although molecules are particularly oriented in the vicinity of the surface layer of the molded article, most of LCP, other than the surface layer, are only spherically dispersed in a matrix of the thermoplastic resin and provided with no reinforcing effect. Specifically, it is not until LCP takes the form of fibers inside the molded article that LCP molecules are oriented in the length direction of a fibrous configuration, so that the resin is provided with the reinforcing effect.
Moreover, when the proportion of LCP increases and the thermoplastic resin not forming an anisotropic molten phase is reduced, LCP this time becomes a matrix in which the thermoplastic resin is dispersed as islands. Such the resin composition is of little utility value since the advantages of the thermoplastic resin cannot be utilized.
Accordingly, methods of producing a molded article containing fibrous LCP provided with a reinforcing effect have been proposed as described in JP-A 5-70700 and JP-A 5-112709, wherein a molding material is first prepared beforehand by stretching and extruding LCP and a thermoplastic resin such as PC at a temperature at which both are molten so that LCP is present as fibers having a high aspect ratio, and then, during molding to form the molded article, the molding material is molded at a temperature at which LCP fails to be molten and only the thermoplastic resin such as PC is molten.
In these methods, however, LCP is stretched and extruded in advance to be oriented as fibers. Alternatively, to form the molded article without any preparation, LCP needs to be formed into fibers by applying a considerable shearing force when a mold is filled up with the resin composition. The former case supposedly results in deteriorated fluidity and limited molding conditions. In the latter case, there are a remarkable influence of the molded article's shape and a possibility of insufficient strength due to partially insufficient fibrous forming.
In consideration of the aforementioned problems, a resin composition has been discovered in which, by blending a specific phosphorus compound such as a pentaerythritol-type triester phosphite, it is no longer necessary to extrude LCP in advance so that it is oriented in a fibrous state, LCP is easily modified into a fibrous state to exhibit a reinforcing effect in molding at such a temperature that the LCP is allowed to flow in a sufficient degree, and thereby the mechanical strength, heat resistance, molding-easiness, etc. of the obtained molded article record quite high values closer to those of LCP (JP-A 7-179743). This resin composition is a material having excellent properties as a molding material for a thin-wall molded article. The most distinct characteristic of this resin composition lies in that LCP is easily modified into a fibrous state by injection molding as described above, an extremely high reinforcement effect which has never been observed is obtained, and therefore the resulting molded article has unique properties and can be a thin-wall one especially excellent in mechanical strength.
However, in the resin composition, there arises a problem that the excellent properties of the molded article expected as described above, are unstable due to a variation of melt-kneading conditions in producing the molding material such as the degree of exhaust from a vent and the degree of contact with ambient air. Therefore, an additive which achieves an excellent effect in a more stable manner has been required.
In consideration of the aforementioned problems, the inventors of the present invention have assiduously studied to obtain a resin composition which can stably exhibit excellent properties as a molding material. As a result, they have discovered that, by injection molding a resin composition, in which one or more kinds of compounds selected from phosphorus oxoacid monoesters and diesters has been added to a resin component comprising a liquid crystal polymer and a thermoplastic polyester resin which does not form an anisotropic molten phase, LCP is easily modified into a fibrous state in the molded article and exhibits an extremely high reinforcing effect which has never been observed, and therefore the resulting molded article has unique properties and an excellent mechanical strength, and these characteristics are stably exhibited in the controllable production conditions of the resin composition. Thus, the present invention has been accomplished.